Polydentate ligand systems

Chelate ring formation may be rate-limiting for polydentate (and especially macrocyclic) ligand complexes. Further, the rates of formation of macrocyclic complexes are sometimes somewhat slower than occur for related open-chain polydentate ligand systems. The additional steric constraints in the cyclic ligand case may restrict the mechanistic pathways available relative to the open-chain case and may even alter the location of the rate-determining step. Indeed, the rate-determining step is not necessarily restricted to the formation of the first or second metal-macrocycle bond but may occur later in the coordination sequence. 

Coordination compounds formed by hard and charged donors with lanthanides, like those of d- and s-block metal ions, are usually thermodynamically stable — especially if coordinated by chelating ligands. Table 9.1 shows a comparison of thermodynamic constants for some key polydentate ligand systems in common with lanthanides. 

Polycatenane complexes, 46 271-273 Polycatenasulfur, 18 304, 305 Polycobalticinium ligand systems, macrocyclic and acyclic, 39 134-140 Polydentate ligands, 32 56 with negative... 

The new features are detailed in this review most using diamide as a model system and illustrated with published data on other extractant systems. Figure 7.1 summarizes the structure of the different extractants cited in this chapter. The mixture of extractants as synergistic systems and extractants, such as the N-polydentate ligands BTP (39, 40), BTBP (41, 42), or bis-malonamide (43), will not be treated here. These extractants have low solubility in alkane and are generally used in chlorinated solvent or octanol or diamide/alkane solution to enhance their solubility. 

If we consider the geometry of the [M(en)3]"+ complex ion, we have further possibilities to consider. Whereas an octahedral complex with six identical ligands can only exist in one form, one with three didentate chelating ligands is chiral and can exist as two (non-superimposable) enantiomers (Fig. 2-7). The incorporation of polydentate ligands into a co-ordination compound may well lead to a rather considerable increase in the complexity of the system, with regard both to the stereochemical properties and any related chemical reactivity. 

The last of the important concepts that we will consider is self-assembly. Most chemists have, at some time in their careers, wondered why molecules cannot just make themselves. The process by which molecules build themselves is termed self-assembly and is a feature of many supramolecular systems. If the molecular components possess the correct complementary molecular recognition features and their interaction is thermodynamically favourable then simply mixing them could result in the specific and spontaneous self-assembly of the desired aggregate. This assumes that there is no significant kinetic barrier to the assembly process. The recognition features within the components may be any of the intermolecular bonding processes mentioned above, but we will be concerned with interactions between transition metal ions and polydentate ligands. 

The original interest in these systems stems from the need to selectively extract heavy alkaline-earth metals from various media.276-280 A number of alkaline-earth compounds with polydentate ligands and anionic ligands... 

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